Powell, G. L. and A. P. Russell 2023. The horns of horned lizards (Phrynosomatidae: Phrynosoma): the long road to their conceptualization as unique lacertilian features with individual identity. Bibliotheca Herpetologica 17(10):86–107. Published October 29, 2023.
Lizards of the genus Phrynosoma are distinguished from all other lizards by the presence of an array of cranial horns, but until recently, these have largely been considered to be homologous only in the aggregate. The terminology used to denote them in the descriptive literature has been inconsistent and not based upon a common definition for these structures. Here we review the terminology applied to the cranial asperities of the species of Phrynosoma, from their first published description to the present day. Early authors noted the presence of “bristles”, “prickles”, “spines”, “horns”, “tuberculous knobs”, “tubercles”, “points”, “thorny eminences”, “stings”, “thorn-shaped projections” and “quills”. Later descriptions used the terms “tubercles”, “protuberances”, “bony processes”, “horns”, “spines”, “tuberosities”, and “rugosities”, at times interchangeably, but there continued to be no generally agreed-upon definition for the asperities to which these terms were applied. A consistent naming scheme for the horns of the species of Phrynosoma is necessary for the formulation and testing of hypotheses of horn homology among its species, and for further investigation of the evolution of these unique features.